Wireless communications devices are an integral part of society and permeate daily life. The typical wireless communications device includes an antenna, and a transceiver coupled to the antenna. The transceiver and the antenna cooperate to transmit and receive communications signals.
A typical personal radio frequency (RF) transceiver or radiolocation tag includes an antenna, radio frequency electronics, and a battery. The antenna, electronics, and battery are often separate components comprising an assembly. Therefore, in many personal transceivers, there can be a tradeoff between battery size and antenna size, between battery capacity and antenna efficiency, and between operating time and signal quality. Antenna performance and battery capacity are related to size, yet personal electronics are typically small while external antennas are unwieldy and often impractical in these applications.
Antennas are transducers for sending and receiving radio waves, and they may be formed by the motion of electric currents on conductors. Preferred antenna shapes may guide the current motions along Euclidian geometries, such as the line and the circle, which are known through the ages for optimization. The dipole and loop antenna are Euclidian geometries that provide divergence and curl. The canonical dipole antenna is line shaped, and the canonical loop antenna is circle shaped.
Antennas generally require both electrical insulators and electrical conductors to be constructed. The best room temperature conductors are metals. As will be appreciated, at room temperature, there are excellent insulators, such as Teflon™ and air. The available electrical conductors are less satisfactory however, and in fact, all room temperature antennas may become inefficient when sufficiently small do due to conductor resistance losses. Thus, it may be important for small antennas to have large conductor surfaces. The material dichotomy between insulators and conductors may provide advantages for small loop antennas: the loop structure intrinsically provides the largest possible inductor in situ to aid efficiency. Capacitor efficiency (quality factor or “Q”) can be much better than inductors so antenna loading and tuning can be realized at low loss when capacitors are used. Loop antennas can be planar for easy printed wiring board (PWB) construction and stable in tuning when body worn.
As will be appreciated by those skilled in the art, a small antenna providing high gain and efficiency would be valuable. Antenna shapes can be of 1, 2, or 3 dimensions, i.e. antennas can be linear, planar, or volumetric in form. The line, circle, and sphere are preferred antenna envelopes as they provide geometric optimizations of shortest distance between two points, greatest area for least amount of circumference, and greatest volume for a least amount of surface area. In small antennas, line, circle, and sphere shapes may minimize metal conductor losses.
Spherical winding has been disclosed as both an inductor in “Electricity and Magnetism”, James Maxwell, 3rd edition, Volume 2, Oxford University Press, 1892. Spherical Coil, pp. 304-308 and as an antenna in “The Spherical Coil As An Inductor, Shield, Or Antenna”, Harold A. Wheeler, Proceedings Of The IRE, September 1952, pp. 1595-1602. The spherical winding approach uses many turns of conductive wire on a spherical core (3 dimensional) and is space efficient. When wound with sufficient turns to self resonate, the spherical winding can have relatively good radiation efficiency for small diameters. The Archimedean spiral can be nearly 2 dimensional and an electrically small antenna of good efficiency.
The thin wire dipole can be nearly 1 dimensional and with an electrical aperture area 1785 times greater than its physical area. The thin wire dipole might offer the greatest gain and efficiency for volume. Thus, there are many advantageous shapes for electrically small antennas, but many antennas do not integrate well in personal communications. For instance, it may be difficult to mount electronic components on some, nearby batteries may shade near fields and radiation on wire loops, the tuning of wound antennas may not be stable when body worn, and whip antennas can be unwieldy. Small antenna design may include tradeoffs in size, shape, efficiency and gain, bandwidth, and convenience of use.
Many personal communication and radiolocation antennas operate on the human body. The human body is mostly water, high in dielectric constant (∈r=≈50), and conductive (δ≈1.0 mho/meter). So in practice, the body worn antenna may have losses and the gain response may not be on the desired frequency, e.g. tuning drift. In particular, antenna electric near fields can be captured by the human body pulling antenna resonant frequency downwards by “stray capacitance.” Antennas using large loading capacitors can have more stable tuning as the body stray capacitance can be small relative loading capacitance. This effect is disclosed in U.S. Pat. No. 6,597,318 to Parsche et al., which also discloses multiple large loading capacitors in series in a loop minimized antenna tuning drift near the human body.
Fixed tuned bandwidth, also known as instantaneous gain bandwidth, is thought to be limited for antennas with small relative wavelength. Indeed, there is a theoretical upper limit, which is known as the Chu-Harrington limit, and notes that the half power (3 dB) fixed tuned gain bandwidth cannot exceed 200(r/λ)3, where r is the radius of the smallest sphere that will enclose the antenna and λ is the free space wavelength. Multiple tuning, such as Chebyschev polynomial tuning, can increase bandwidth above this by up to 3π for infinite order tuning. In practice, double tuning can increase bandwidth by a factor of 4. In multiple tuning, the antenna may become one pole of a multiple pole filter, and the filter may be provided by an external compensation network.
If light propagated at a lesser speed, all antennas would be electrically larger and with better bandwidth for size. U.S. Pat. No. 7,573,431 to Parsche discloses immersing small antennas in nonconductive materials having equal permeability and permeability, i.e. (μ=∈)>1, in order to aid bandwidth at small physical size. This approach may identify that the boundaries of isoimpedance magnetodielectric (μ=∈) materials are reflectionless to waves entering and leaving free space and air. The approach also may show that the speed of light is significantly slowed in isoimpedance magnetodielectric materials. Thus, these antennas can have good bandwidth inside (μ=∈)>1 materials as they become electrically larger without physical size increase. Except for refraction, isoimpedance magnetodielectric materials are invisible materials at frequencies for which the isoimpedance property exists, as such materials have negligible reflections to vacuum and air.
In addition to the design concerns discussed above in regards to power efficiency and performance, there has been a desire to miniaturize wireless communications device for several reasons. Indeed, certain applications, for example, wireless tracking devices, place a premium on the miniaturization. In particular, reduced packaging may enable the wireless tracking device to be installed without substantial modification to the tracked host. Miniature radiolocation tags are useful for diverse applications, such as wildlife tracking, personnel Identification, and for rescue beacons. Of course, the miniaturization of the wireless tracking device also aids in subterfuge if the device was installed surreptitiously. One approach is disclosed in U.S. Pat. No. 6,324,392 to Holt, also assigned to the present application's assignee. This approach includes a mobile wireless device that broadcasts a wideband spread spectrum beacon signal. The beacon signal summons assistance to the location of the mobile wireless device.
Yet another approach is disclosed in U.S. Pat. No. 7,126,470 to Clift et al., also assigned to the present application's assignee. The approach includes using a plurality of radio frequency identification (RFID) tags for tracking in a network including a plurality of tracking stations.
Yet another approach is provided by the EXConnect Zigbee Chip Antenna Model 868, as available from the Fractus, S.A., of Barcelona, Spain. This chip antenna has a compact rectangular form factor and includes a monopole antenna. The chip antenna may be installed onto a printed circuit board (PCB). A potential drawback to this approach is that the PCB may need to be tuned for efficient operation for each application.
Another approach may comprise a wireless device fashioned into a business card form factor and includes a pair of paper substrates. The wireless device includes a pair of lithium ion batteries, and wireless circuitry coupled thereto. Conductive traces are formed on the paper substrates, for example, 110 lb paper, by screen printing conductive polymer silver ink thereon. The wireless device also includes a 1/10 wavelength loop antenna. A potential drawback to this wireless device is that the separated antenna and wireless circuitry may result in reduced battery life and weaker transmitted signals.
An approach may comprise a wireless tracking device fashioned into a bumper sticker form factor and includes a segmented circular antenna, a battery, and wireless circuitry coupled to the battery and antenna, each component being affixed to a substrate. Again, this wireless tracking device may suffer from the aforementioned drawbacks due to the non-integrated design.